Conrad Pitts, a lawyer for Sunshine Mills, said 80 percent of the tainted biscuits were sold by Wal-Mart, under the Ol' Roy brand. Pitts said the company had produced about 24 truckloads of biscuits with the contaminated gluten, and that the majority of product was large biscuits. Pitts said wheat gluten accounts for less than 1 percent of the total weight of the biscuits.

Until last week, when moist cat treats, dog jerky and a type of dry cat food were added to the recall, it had been limited to wet pet food sold under a variety of brand names.

Menu Foods, which last month recalled more than 90 brands of its "cuts and gravy" pet food, said yesterday that it had extended the period of time covered by its recall to include food made after Nov. 8. The company, based in Ontario, initially recalled only food made from Dec. 3, to March 6.

The company also added 20 additional varieties of those brands to the recall list yesterday.

Menu Foods said it acted after a supplier, ChemNutra of Las Vegas, recalled all wheat gluten it had imported from Xuzhou Anying Biologic Technology Development Co. of Wangdien, China. ChemNutra said Wednesday that the FDA had found melamine in the gluten. The FDA said it was now testing all wheat gluten from China.

The Chinese government said yesterday that no wheat gluten had been exported to the United States or Canada. Xuzhou Anying denied it had ever shipped wheat gluten to either country.

"We are a trading company and don't manufacture the product," said Mao Lijun, the company's general manager. But Michael Rogers, director of the Division of Field Investigations for the FDA, said records showed that the tainted gluten came from China.

Rogers and Sundlof said the gluten did not enter the human food supply. The agency said it was still investigating how the melamine got into the gluten, and that it had notified all companies that had received it.

Though melamine has been found in the food and in the urine and kidneys of pets that have eaten the food, officials and scientists are not sure whether the chemical caused pets to get sick.

Melamine, which is also used as a slow-release fertilizer, is generally not known to be toxic. Some theories are that it might act as a marker for another unknown toxin that causes renal failure in pets, Sundlof said, or that cats and dogs are extremely sensitive to melamine.

"We still have a lot of work to do to understand why melamine is involved, as it is a relatively nontoxic substance," Sundlof said. "We are relatively certain that there is a connection here someplace."

The FDA said it had received more than 12,000 complaints about pet food since the recall, as many as it usually gets in a two-year period on all topics combined. It has confirmed 16 deaths.

"We have no good information what that final number might be," Sundlof said. "It will take awhile for us to get there." The College of Veterinary Medicine at Iowa State University is investigating 43 suspected cases of pets that died from eating tainted food; the deaths of 18 are consistent with ingestion of a toxic substance, said Patrick Halbur, executive director of the Veterinary Diagnostic Lab at Iowa State.

Halbur and Grant Maxie of the University of Guelph in Ontario, which is also investigating the cause of the illnesses and deaths, said it would probably take months to determine what made the pets sick.

In Chicago, Sen. Richard J. Durbin, an Illinois Democrat, called for a hearing to question FDA officials. He also called for standardized federal regulations and inspection requirements for pet food.

The recall

Menu Foods, a major manufacturer of nearly 100 store- and major-brand pet foods, announced the first recall March 16. Other companies, including Hill's Pet Nutrition Inc., Del Monte Pet Products and Nestle Purina PetCare Co., have since recalled some of their products as well.

Yesterday, dog biscuits made by an Alabama company and sold by Wal-Mart under the Ol'Roy brand were recalled.

The government has blocked wheat gluten imports from the Xuzhou Anying Biologic Technology Development Co. in the eastern city of Xuzhou, saying they were tainted with melamine, which has been found in some pet food.